Electric arc device



P. FREEDMAN 1,959,419

ELECTRIC May 22, 1934.

ABC DEVICE Filed July 1, 1929 Patented May 22, 1934 UNITED STATES ELECTRIC ARC DEVICE Paul Freedman, London; England Application July 1, 1929, Serial No. 375,0'76 In Great Britain July 5, 1928 The present invention relates to electric arc discharge devices, which may be used as electric oscillators, as rectifiers for alternating currents,

, u or as amplifiers.

The invention is embodied in a device having an anode, a cathode and a grid electrode, arranged as hereinafter described, sealed into a bulb of glass or silica containing inert gas or N gases at a suitable pressure. It may also possess an auxiliary starting cathode.

The primary cathode is either in the form of a filament or in the form of a sphere or block of suitable proportions mounted on a stalk whose efiective cross section, to minimize conduction losses, is made relatively small.

The anode is in the form of a sphere, plate, block or ring of suitable proportions mounted on a stalk of relatively small cross section.

, The grid electrode may be of any convenient shape, but is preferably in the form of a sphere,

plate or block mounted on a stalkof relatively small cross section.

All the electrodes and their stalk supports are composed of conducting material of relatively high melting and boiling points, so as to ensure long life under normal conditions of operation of the device. Tungsten is an eminently suitable refractory electrode material, but in certain types of the device, wherein relatively low working temperatures of the electrodes prevail, ma-

terials of considerably lower melting and boiling points may be conveniently used.

In the case where a filamentarycathode is employed, the said cathode may contain or be 5 coated with materials whose function is to cause an increase in the electron emissivity of the surface of the said cathode. Thus for instance a filamentary tungsten cathode provided with a sleeve of thoria or of thoria and tungsten mixed in suitable proportions threaded over it and covering the whole or a portion of its length, may be employed. Or again, a filamentary cathode of platinum or nickel, coated with oxides of calcium, strontium and barium used either 4,5severally, or mixed in any desired proportions, may be used. Other modifications of cathode construction and composition readily suggest themselves.

Although a number of refractory conducting 5 substances are suitable for the anode, grid or cathode, sphere, ring block or plate portion, fused tungsten is particularly advantageous.

The electrodes are so proportioned that, when .the device is operating normally, the cathode is at a temperature at whichit is capable of a copious emission of negative electrons, while the anode and grid are at a temperature sufficiently high to enable them, jointly to emit as many positive ions as may be required to maintain, by bombardment, the correct working temperature of the cathode.

In one particular case, when the cathode is a filament, it is possible to use auxiliary heating current through the filament-securing the requisite cathode temperature during the working of the device.

In another particular case, when an auxiliary starting cathode is employed, a discharge between the main and the auxiliary cathode may be employed during the working of the device in obtaining correct temperature conditions at the cathode.

The said auxiliary starting cathode may be of the form and composition already mentioned with respect to the filamentary cathode.

The upper limit of anode and grid temperatures, apart from the question of electrode life, which must decrease with increased temperature of the electrodes, is determined by the use to which the device is put. Thus, if the device is used for rectification, the anode temperature must be sufiiciently low to prevent appreciable emission of negative electrons from the anode; a necessary condition for the prevention of reverse currents.

The electrodes are so situated that the anode is between the grid and the cathode. The anode should not, however, completely screen the grid .and the cathode from each other.

The bulb into which the electrodes are sealed is carefully evacuated and filled with a suitable gas such as nitrogen argon or neon, or an admixture of these gases at a suitable pressure. In certain cases, vapour of metals of low boiling point, such as mercury, rubidium or caesium may be employed in addition to or instead of the gases above mentioned.

When the device is operating normally, the grid is positive with regard to the cathode and an arc discharge possessing a negative volt ampere characteristic is maintained between them.

The discharge which passes between the anode and the cathode may possess either a positive or a negative volt ampere characteristic, dependingupon the working conditions but it is most commonly negative.

There is an interaction between the gridcathode and anode-cathode discharges, a variation of conductivity in one discharge causing a variation in the conductivity of the other, the

nature and magnitude of variation being dependent upon the initial variation and the original values of potential and current in both discharge circuits.

The grid-cathode discharge operates at relatively high voltage and relatively low power, whilst the anode-cathode discharge operates at relatively low voltage and relatively high power.

In order that the invention may be better understood, it will now be described with reference to the accompanying drawing in which:-

Fig. 1 shows a type of the device comprising.

an anode '7 in the form of a bead of refractory metal mounted on a stalk of a similar material, a grid electrode 8 in the form of a refractory metallic bead upon a refractory metallic stalk and a composite cathode 9 consisting; of apartially spiralized and partially straight tungsten filament over the straight portion of which is threaded a tube composed of a mixture of tungsten and refractory oxide or oxides, such as thoria, all electrodes being included in a gasfilled vitreous envelope.

The device is started by passing a current through the cathode and raising the spiralized portion of same to a bright incandescence. The requisite temperature of the cathode and the ionization of the adjacent gases being thus attained, the heating current through the cathode is interrupted and a discharge is formed between the anode and the spiral portion of the cathode. This discharge gradually shifts on to the refractory tubular portion of the cathode and when a steady state has been obtained the discharge between the grid electrode and the cathode hot spot may then be formed.

Fig. 2 shows a modified form of the device comprising a gas-filled vitreous envelope having therein a cathode 10 in the form of a spiralized filament having a metallic core and superficially coated with oxides calculated to cause an increase in its electron emissivity, an anode 11 in the form of a refractory metallic bead mounted on a refractory metallic stalk and a ,grid electrode 12 in the form of a refractory metallic plate mounted on a stalk of similar material. The mode of operation of this type of device is identical with that previously described except that it is advantageous to maintain the cathode heating current throughout the operation of the device.

Fig. 3 shows another form of the device comprising a gas-filled vitreous envelope wherein are situated a cathode 13 consisting of a bead of refractory metal mounted on a refractory metallic stalk, an anode 14 in the form of a ring composed of a refractory metal mounted on a stalk of a similar material, and a grid electrode 15 in the form of a bead of refractory metal mounted on a refractory metallic stalk. device is started by a spark, glow or corona discharge.

Fig.4 shows another type of the device, similar to that shown in Fig. 3 but containing a vaporizable metallic pool 16. Metallic vapour is generated during operation of the device in virtue of heat communicated to the metallic pool from the arc region and the heated electrodes, and this metallic vapour substantially modifies the characteristics of the device.

- a Fig. 5 shows a type of the device suitable for a large power output. This type of device comprises a gas-filled vitreous envelope wherein is situated a grid electrode 17 in the form of a refractory conducting block mounted on a stalk of refractory metal, an anode 18 in the form of a refractory metallic bead mounted on a refractory metallic stalk, a cathode 19 also in the form of a refractory metallic bead mounted on a refractory metallic stalk, and an auxiliary starting cathode 20 in the form of a spiralized metallicfilament superficially coated with oxides calculated to increase its electron emissivity. The mode of operation of the device will be readily understood from the preceding descriptions.

Fig. 6 shows a group of typical characteristic curves of a device constructed according to the present invention.

What I claim and desire to secure by Letters Patent of America is:

In an electric arc discharge device having a vitreous envelope containing an inert atmosphere, a refractory filamentary cathode part of which isspirally coiled, a refractory tube fitted over another part of the cathode, a tungsten bead suspended on a stalk at some distance away from the cathode and constituting a grid electrode, a tungsten .bead suspended on a stalk between said grid and said cathode constituting the'anode and which is of comparatively small diameter so that the'said anode does not screen the grid from the cathode completely and means for supplying current to all of said electrodes and for passing current through said filamentary cathode during starting to raise same to incandescence.

PAUL FREEDMAN. 

